Illumination is essential to human civilization and it has very long history. The material, energy efficiency and illumination quality evolve with technology. Today electricity is the primary energy form for illumination. About 20% of the electricity power in the world is used for illumination. Comparing to commercial lighting, industrial lighting and other fields, portable lighting is a relatively small market in illumination. But people still have persistent needs for portable lighting in many areas, such as outdoor sports (hiking, camping, hunting), residential activities (security, back-up lighting), law enforcement and military (night patrol, special operations), industry (machine maintenance, part inspection), etc. The annual revenue of portable lighting businesses in US is more than 4 billion dollars now, which keep tens of thousands of people employed. And there is still plenty of room for innovations and improvements in the product design of portable lights.
The major categories of portable light include flashlight, lantern, headlamp, bicycle light, etc., within which flashlight is the biggest and dominant category. Flashlight usually covers the portable lights that are powered by batteries with handhold body for projecting light to a specific direction. Its power source could be non-rechargeable batteries, such as AA, AAA alkaline models or CR123 Li—Mn model, or rechargeable batteries, such as 18650, 14500 lithium-ion models, etc.
The types of light source of portable lights include incandescent light bulbs, light-emitting diodes (LED) and gas discharge lamps. LED is becoming more and more popular recently for its higher efficiency, longer life, better stability and adjustability. However it also has drawbacks. LED requires driving and protection circuits to work which increase the cost. And as the brightness increases, the tiny LED chip generates lots of heat which requires proper heat dissipation structure. And in addition, the control of the light color or spectrum of LED has to be handled carefully.
LED has the feature of generating very narrow color spectrum, whereas the traditional light sources such as tungsten lamp can generate continuous spectrum over the full range of the visible light wavelength. People have to mix the different single-color lights generated by LEDs to produce white light that is suitable for daily illumination. The most common way is using a green-light LED chip (i.e. InGaN) on a phosphor substrate. The green light from the LED combines with the stimulated yellow light from the phosphor to provide the white color output. By adjusting the ratio of the individual colors, people can tune the output color to the specific correlated color temperature with satisfactory color rendering index.
On the other hand, by combining the basic color lights, such as red, green and blue, we can generate any visible light color on the CIE chromaticity diagram, not only the white light. While the technique of colorful illumination is widely applied in other fields such as architecture lighting, it's also desirable in portable lighting. For outdoor users, red light is preferred for reading maps in dark environment, blue light is better for tracing blood and green light is welcomed by hunters as it cannot be seen by some big games. Besides, people may just want to have different colors of light due to different individual preference, mood or environment.
For the same user, his/her mood, the environment and application vary over time. Therefore, it is desirable to have a portable light for which the light color can be changed by the user conveniently instead of being preset by the manufacturer.
LED or other solid-state light emitting devices can not only generate visible light but also infrared and ultraviolet lights. By the same method of mixing different basic colors in visible light, we can compose synthetic spectra from the individual light sources. Therefore, the output spectrum can be controlled to suit people's special need.
U.S. Pat. No. 7,293,893 (Paul Kim, 2004) discloses a flashlight that includes at least two different output wavelengths in addition to the primary light source. And U.S. Pat. No. 7,896,518 (Danny Holmes, 2008) describes a LED flashlight that produces at least three different colors and is operated by two separate switches. Both the two inventions bring additional colored lights into a flashlight in addition to the common white color light. However, they didn't mention the combination of different colors to generate other colors and the control means of their design are separate switches that work as function/color selectors. As the conclusion, the manipulation of colors in the two prior arts is limited to selection of basic colors instead of adjusting the output light color in the full-color range and in a stepless manner.
U.S. Pat. No. 6,016,038 (George Mueller, 1997) and its related patents assigned to Color Kinetics, Inc. describe the way of controlling LEDs to alter the brightness and color of the generated light. Along with their success in business, their invention has been applied in architecture illuminations and other areas. But none of the core elements of a portable light that allows the user to adjust color conveniently were included in their disclosure due to the difference between a portable light and architecture lighting systems.
First, a portable light (including flashlight) has very compact size for every day carry in pocket or handbag. If in cylindrical shape, its diameter is usually less than one inch which would be at least five times smaller than a regular architecture light. The circuit board of a flashlight is typically of the size of a quarter-coin which is difficult to design in order to maintain high energy efficiency. Thus the system oriented for architecture lights won't suit the portable lights automatically.
Second, a portable light is usually powered by batteries and so the electrical circuit system must take a very different form to that of the architecture lights which are connected to the 110V electricity network. The output voltage of a battery varies as it drains and a set of batteries can usually only support the flashlight for a few hours before they are drained up. As the working voltage of LEDs of different colors are different (for red color it's usually much lower than green and blue), if the voltage applied on the LEDs is decreasing, people will see the output color becomes weaker and more yellowish. Therefore, the power management is one of the biggest challenges in developing a portable light with color adjustability while it's not an issue for cabled lighting device.
And the last, the working range of the two illuminations are different. The architecture light usually sheds lights onto objects sitting far away from the light, typically beyond one yard. The portable lights would penetrate far distances up to hundreds of yards in special applications but mostly they have to illuminate very close targets such as within one foot. As the target is closer, the spatial color discrepancy would be more visible. Therefore, for the portable light with color adjustability, the different basic colors should mix more uniformly and thoroughly to avoid rainbow-type color deficiency. It requires special optical design.